Avatar put me to sleep, but I admire James Cameron's thirst for new film technologies. Now I…
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Why? Basically, NASA didn't have enough time to go through all the testing of the camera before launch. Makes some sense, you wouldn't want a faulty cam when you're on vacation let alone Mars. According to NASA:

The possibility for a zoom-camera upgrade was very much worth pursuing, but time became too short for the levels of testing that would be needed for them to confidently replace the existing cameras."

NASA assures us that the cameras on board (Mastcam 100, Mastcam 34) will be enough to complete the mission. Guess there's always next time, or Avatar 2. [NASA via The Register via DVICE]